Question about Loadpull using ADS

[bubuta77]

Hi all
I have some questions about loadpull utility:
1.If my transistor not stable at tested frequency do i need to make loadpull with stabilizing network?
2. How i find the correct Pin for loadpull?
Thanks'all

 

[BigBoss]

If the Power Amplifier is not stable, you should make it satable first then you will able to do Load Pull.
Pin is the function of the Gain, so that Maximum Ouput Power is well defined first then extract the Gain to able to find Pin.
Maximum ouput power can be defined either being as Max. Saturated Power or Power where the Gain is dropped by 1dB.But initially, P1dB is selected for a good forecast then you may go further.
P1dB output power should normally be either forecasted or simulated.

 

[bubuta77]

Thanks
by the way i am able to run Loadpull with unstable transistor, but with stabilizing network i get different result.
I can't define the maximum output power cause i use a loadpull to find optimum impedance for maximum power: for example i get 10W with optimum impedance x+y*j ohm but if i increase pin i still probably get more power for different optimum impedance and it can continue till transistor will badly saturate.

 

[BigBoss]

Thanks
by the way i am able to run Loadpull with unstable transistor, but with stabilizing network i get different result.
I can't define the maximum output power cause i use a loadpull to find optimum impedance for maximum power: for example i get 10W with optimum impedance x+y*j ohm but if i increase pin i still probably get more power for different optimum impedance and it can continue till transistor will badly saturate.

That's why you should resolve your stability problem then you can check your amplifier's optimum load condition.Since your amplifier is unstable, all mesaurements will be erroneous.
Second thisn that you should consider that is Maximum Ouput Power definition. Will it be hardly saturated power or only P1dB compression.While you check your amplifier, observe its gain simultaneously.
I think the best way to determine the optimum load for this amplifier is to get a half of the typical power and define the optimum load for this condition and then search the other possibilities for that.

 

[bubuta77]

Thanks, was helpful.
Just last question...
i build stabilizing network using S parameters and all looks fine, BUT .. when i use the LSSP my K and mu are near zero at all power levels.. does it mean i need to design stabilizing network using LSSP? (I know that for power amplifiers S parameters differ for power level but i always thought and saw at all books that stabilizing networks build using small signal....)

 

[BigBoss]

Thanks, was helpful.
Just last question...
i build stabilizing network using S parameters and all looks fine, BUT .. when i use the LSSP my K and mu are near zero at all power levels.. does it mean i need to design stabilizing network using LSSP? (I know that for power amplifiers S parameters differ for power level but i always thought and saw at all books that stabilizing networks build using small signal....)

If K factor is less than unity, check also mu factor.It essentially doesn't mean that the ampifier will absolutely oscillate when K factor is less than unity.